A catalyst for purifying exhaust gases, in general, is formed by forming a porous loading layer, which is made of a heat-resistant inorganic oxide, and the like, on a surface of a catalytic support substrate, which exhibits heat resistance, and loading a catalytic metal on this loading layer. The catalyst for purifying exhaust gases, by the action of the catalytic metal, converts harmful nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO), which are included in exhaust gases, into harmless nitrogen, carbon dioxide and water.
It has been known that the purifying performance of exhaust gases, which results from catalytic metals, receives the influence of temperatures strongly, and accordingly the purification is generally carried out at temperatures of 300° C. or more. Because of this, like immediately after starting engines, the temperature of exhaust gases is low, the catalytic activity of catalytic metals is low, and so the purification of exhaust gases has been difficult.
As the method of improving the purifying performance and ignition performance of catalyst for purifying exhaust gases, a method of letting one catalyst for purifying exhaust gases possess two different characteristics has been known widely. Such a catalyst for purifying exhaust gases is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 62-68,542 and Japanese Unexamined Patent Publication (KOKAI) No. 62-65,746.
However, these catalysts for purifying exhaust gases are such that the durability has not been sufficient.